You can try to download the Moonlight extension and bump the maxVersion from 4.0.* to 5.*
The extension is signed to protect the extension against changes, so you need to remove the META-INF folder that stores the data to verify the signing.

I changed the maxVersion to 5.0* and removed META-INF folder but I am getting the same error "extension may be corrupt". What am I missing?
I also tried installing compatibility Reporter addon, but it does not let me install moonlight. I checked extensions.checkCompatibility in about:config, and everything is set to false. Please help

riteshbafna,
I had the exact same problem. I'm assuming you had created a new folder where you extracted all the files from the original .xpi file. It seems it doesn't like it if you zip the entire folder (because when unzipped it produces a folder, not the individual files). So I just opened the folder (after modifying the .rdf file and deleting the META-INF folder), did "Select All", right-clicked and chose "Compress", and renamed the resulting .zip file to .xpi. Voila!